Tinnitus is the perception of sound in the human ear in the absence of corresponding external sound(s). Tinnitus is considered a phantom sound, which arises in the auditory system. For example, a ringing, buzzing, whistling, or roaring sound may be perceived as tinnitus. Tinnitus can be continuous or intermittent, and in either case can be very disturbing, and can significantly decrease the quality of life for one who has such an affliction.
Tinnitus is not itself a disease but an unwelcome symptom resulting from a range of underlying causes, including psychological factors such as stress, disease (infections, Menieres Disease, oto-sclerosis, etc.), foreign objects or wax in the ear and injury from loud noises. Tinnitus is also a side-effect of some medications, and may also result from an abnormal level of anxiety and depression.
The perceived tinnitus sound may range from a quiet background sound to a signal loud enough to drown out all outside sounds. The term ‘tinnitus’ usually refers to more severe cases. A 1953 study of 80 tinnitus-free university students placed in a soundproofed room found that 93% reported hearing a buzzing, pulsing or whistling sound. However, it must not be assumed that this condition is normal—cohort studies have demonstrated that damage to hearing from unnatural levels of noise exposure is very widespread.
Tinnitus can, to date, not be surgically corrected and since, to date, there are no approved effective drug treatments, so-called tinnitus maskers have become known. These are small, battery-driven devices which are worn like a hearing aid behind or in the ear and which, by means of artificial sounds which are emitted, for example, via a hearing aid speaker into the auditory canal, to thereby psycho acoustically mask the tinnitus and thus reduce the tinnitus perception.
The artificial sounds produced by the maskers are often narrow-band noise. The spectral position and the loudness level of the noise can often be adjusted via for example a programming device to enable adaptation to the individual tinnitus situation as optimally as possible. In addition, so-called retraining methods have been developed, for example tinnitus retraining therapy (Jastreboff PJ. Tinnitus habituation therapy (THI) and tinnitus retraining therapy (TRT). In: Tyler R S, ed. Handbook of Tinnitus. San Diego: Singular Publishing; 2000:357-376) in which, by combination of a mental training program and presentation of broad-band sound (noise) near the auditory threshold, the perceptibility of the tinnitus in quiet conditions is likewise supposed to be largely suppressed. These devices are also called “noisers” or “sound enrichment devices”. Such devices or methods are for example known from DE 29718 503, GB 2 134 689, US 2001/0051776, US 2004/0131200 and U.S. Pat. No. 5,403,262.
Another system is known from WO 2004/098690, wherein spatial filtering is used in a binaural hearing aid system, i.e. a hearing aid system containing two hearing aids, wherein the input signals to the two devices are manipulated in such a way that the perceived direction of origin of the input signal is altered in a number of different ways. It is mentioned that the spatial filtering may be obtained by changing the spectral properties of the incoming sound signal along with a manipulation of the phase and signal level of the incoming sound signal. For example, the signal level is manipulated in dependence of the input signal level in resemblance with an automatic gain control circuit. It is alleged that such a system may provide relief for, and even treatment of, tinnitus.
From U.S. Pat. No. 6,047,074 is known a hearing aid comprising a signal generator for the provision of a noise signal employable in tinnitus therapy. The disclosed hearing aid also includes a signal analysis stage by which the input signal of the hearing aid may be analyzed. The input signal spectrum can then be analyzed in order to find out if an adequately high signal level is present in the frequency range that is needed for tinnitus therapy. If this is the case, then the signal generator is not activated. If however, the input signal level is low, then the signal generator is activated. The decision to apply the tinnitus therapy signal is thus merely based on the input signal of the hearing aid.
Although present day tinnitus maskers to a certain extent may provide immediate relief of tinnitus, the masking sound produced by them is very monotonous and therefore unpleasant for the user of such a masker. Investigations show that tinnitus is a condition that requires long term treatment in order to achieve good results. However, the listening to highly monotonic masking sounds during such a long time may be a severe annoyance to a user of such a masker.
For many people, the known maskers will not provide any long term relief of tinnitus. Recent research conducted by Del Bo, Ambrosetti, Bettinelli, Domenichetti, Fagnani, and Scotti “Using Open-Ear Hearing Aids in Tinnitus Therapy”, Hearing Review, August 2006, has indicated that better long term prospectives for tinnitus relief may be achieved if so-called habituation of tinnitus is induced in a tinnitus sufferer by using sound enrichment by sound from the ambient environment. The rationale behind habituation relies on two fundamental aspects of brain functioning: Habituation of the reaction of the limbic and sympathetic system, and habituation of sound perception allowing a person to ignore the presence of tinnitus. While tinnitus maskers emit sounds that either partly or completely cover the perceived sound of tinnitus, Del Bo, Ambrosetti, Bettinelli, Domenichetti, Fagnani, and Scotti, suggest the use of environmental sounds amplified by a hearing aid or by application of artificial sounds, such as band limited noise.
However since, traditional sound enrichment often has to be used for many months in order to achieve the habituation of a person's perception of tinnitus, the monotony of the used sound signal may be annoying and uncomfortable for some users to listen to.